Vehicles employ a number of facilities, such as road side parking, parking in lots and toll roads, which are available for a fee. Electronic toll systems are known which monitor use of such facilities by vehicles so as to compute and levy an appropriate fee.
A state of the art electronic toll system is described by Dieter Rittich and Konrad Zurmuhl in "Zukunftige Automatische Gebuhrenerfassung fur den StraBenverkehr", Sonderdruck aus ntz Bd. 46 (1993), Heft 4. The Rittich et al system employs the ALOHA data communication protocol in order to provide periodic transmission to a vehicle tag at a randomly determined point of time within each period.
U.S. Pat. No. 4,075,632 to Baldwin et al describes a backscattering tag in which the antenna, when in transmitting mode, is switched between the ground and an open circuit.